Radium isotope contribution of temporal dynamics assessment of surface water infiltration into aquifers

Sabine Veuille

Residence time of surface water during infiltration into an aquifer influences many biogeochemical processes and indirectly the quality of groundwater. These processes lead to the natural attenuation of contaminants. Many municipalities uses these natural processes mostly named Induced bank filtration (IBF) as a pretreatment of surface water. Quantifying the residence time of infiltrated surface water is necessary prior information to anticipate quality problems in these systems. In the case of the IBF, due to the seasonal hydro-climatic regime and variable pumping sequences, induced surface water infiltration is a heterogeneous process poorly described by conventional hydrodynamic approaches. Natural radioisotope tracers provide hope for a better understanding of the complex dynamics of these infiltration processes.

Radium isotopes have unexploited potential for tracing the dynamics of infiltrating surface water. Radium is a geogenic element with four major isotopes spanning ideal half-life ranges (4 days to 1600 years) for and understanding the diversity of residence times of seepage water in the subsurface at FSB sites. Its potential to characterize the mixing process of fresh and salt water, to date seawater or to estimate mass balances in coastal estuaries has been widely demonstrated. In this context, using radium isotopes presents many challenges. In freshwater, radium is affected by adsorption-desorption processes that depend on ionic strength, pH and redox conditions of the water. The consequences of these processes have implications for the interpretative framework, for sampling on manganese fibers and for the measurement of the element by alpha-spectrometry. To answer these questions, a case study was conducted over a two-year period on a site of (FSB) in Quebec and allowed the collection of 115 samples. This study also relied on laboratory tests to size the sampling yield and to evaluate the accuracy of the measurement.